International Conference on Computing Technologies (ICONCT ... · DC-DC converter is an electronic circuit that converts a DC source of a certain voltage level to another voltage

International Conference on Computing Technologies (ICONCT’17)

Organized by Department of Computer Science and Engineering & Information Technology 72

Investigation of an Energy Storage Cell based

Switched-Capacitor DC-DC Power Converter for PV

System Employing PSO based PI Control Technique

M.Santhoshini1,Dr.R.Pon Vengatesh and Dr.S.Edward Rajan

3

1 PG scholar,

2Assistant Professor (Sr. Grade),

3Professor,

Department of Electrical and Electronics Engineering,

Mepco Schlenk Engineering College (Autonomous), Sivakasi, Tamil Nadu, India.

Email: [email protected];[email protected];[email protected]

Abstract—Now-a-days the demand of renewable energy as an electrical source is increased due to the availability of limited amount of fossil fuel and their environmental pollution. In this research paper, anenergy storage cell based convertercircuitwith combination of a Switched-mode DC–DC converter and a Switched-

Capacitor(SC) converter model is incorporated for Photo-Voltaic

(PV) system.The proposed DC-DC converter has been modelled and simulated using Matlab-simulink environment for obtaining high gain. Moreover,PV system is conveniently interfaced with proposed power converter for maximizing the solar energy yield and isanalyzed by employing conventional Perturb & Observe (P&O) algorithm under various operating conditions. The Particle Swarm Optimisation (PSO) based PI controller is also employed to achieve voltage regulation. Moreover, the battery charging technique has been adopted for charging Lead acid battery in standalone PV system applications.

Index terms—Switched-Capacitor(SC), Photo Voltaic(PV), High efficiency,PSObased PI controller,Energy Storage cell based Switched Capacitor.

I. INTRODUCTION

The usage of fossil energy has affected the environment on a global scale, such as environmental pollution and greenhouse effect.Traditional fossil energy is not renewable hence it is being faced with the problem of energy shortage. Therefore, it is necessary to develop new energy which is clean and renewable to replace fossil energy. Solar energy is promising, Photo Voltaic (PV) power generation as the utilization method have been applied on a large scale. On the other hand, the output voltages of PV are commonly ranged from 25 to 45 V, which are lower than the bus voltage[1-2]. In order, to boost the output of PV, DC-DC converter with a high voltage gain is required. A high voltage gain can be obtained by increasing the transformer turns ratio in isolated converters. However, the usage of large turns ratio maylead to a large leakage inductance and the efficiency of the converter get reduced but the voltage stress of the switch willget increased. Among the non-isolated DC-DC converters, the boost converter is preferred for voltage step-up. The duty cycle will approach to unity when the output voltage is much higher than

the input voltage. Thus, the current ripple of the inductor and turn-off current of the power device are large, which results in large conduction loss, switching loss, and thus low efficiency. By cascading another boost converter, a high voltage gain can be easily obtained. However, too many components are required, leading to high cost and low overall efficiency.

Fig.1. Block diagram of the proposed energy harvesting system.

The switched capacitor converter can obtain maximum voltage gain, but the input current is pulsatingand the output voltage is not regulated.By addition of SC structure along with switching mode DC-DC converter, the voltage gain can be severely increased. The non isolated switching mode DC-DC converter has excellent output regulation, but high voltage gain cannot be obtained. This paper proposes a combining method of SC converter and the switching mode DC-DC converter. The principle behind this paper is the inductor get charged, when switch is turned ON, and the capacitors which are connected in series is used to supply the load, and then the inductor releases energy to charge multiple capacitors in parallel, when the switch

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is turn OFF.Thus, the duty cycle is increased, efficiency get decreased.

II. BASICDESIGN OF SC ADOPTING INDUCTOR

ENERGY STORAGE CELL

The voltage of the SC can be regulated by adjusting the

duty cycle of the switch.

Fig.2. SC structure with a Inductor Energy Storage Cell.

(a) Boost (b) Buck–boost.

The circuit shown in Fig. 2(a) is like a Boost converter,

whereas the circuit shown in Fig. 2(b) resembles a Buck–Boost converter. Hence, the SC is called the Boost capacitor when it is in parallel with the switch, and the SC is called the Buck–Boost capacitor when it is in parallel with the inductor. Diode D is used to prevent capacitor C from being shorted [see Fig. 2(a)] or being in parallel with the input voltage source [see Fig. 2(b)] when Q is on. Obviously, the positions of the diode and the SC can be exchanged.

A. Proposed DC-DC Converter

DC-DC converter is an electronic circuit that converts a DC

source of a certain voltage level to another voltage level [3-4].

Fig.3. Proposed DC-DC converter.

By combining the Fig.2(a) and Fig.2 (b), Nonisolated high step-

up DC–DC converter with Single-Inductor-Energy-Storage Cell-

based SCs (SIESC-SCs) is derived.To obtain a high voltage gain,

the SCs should be connected in series as many as possible when

switch Q is ON. Moreover, the polarities of the capacitors to be

connected in series should be different at the connection

point.The preferred DC to DC converter circuit consists of a

switch and an energy storage devices like inductors or

capacitors[5-6].

B. Principleoperation of DC-DC converter

In this converter, one of the SCs is the boost capacitor,

and the other one is the Buck–Boostcapacitor. The two capacitors

and the voltage source are connected in series to supply the

load[7-8]. Thus, the voltage gain of a high step-up converter with

SIESC-SCs derived from Boost/Buck–Boost based converter is

yy

y

yg

o

DD

D

DV

VM

1

21

11

1(1)

In the converter shown in Fig. 3, part of the energy is directly transferred to the loadfrom the input source when the switch conducts. In Fig 3 shows the proposed converter which is the combination of the Switched Capacitor (SC) converter and the switching-mode DC–DC converter. C1 and C2 are connected in series through Switch as shown in Fig 3. The series-connected capacitors are connected to the output filter capacitor Cf and load resistor RL through diode Do. Do is used to prevent the output from being in parallel with one of the SCs when the power switch S is turned off and the diodes conduct. The basic concept is introducing multiple capacitors into the switching-mode DC–DC converters.When the switch is OFF the energy released from the inductor L is used to charge the capacitors in parallel. When the switch is ON, the capacitors are connected in series to supply the load. Thus, the duty cycle get decreased and voltage gain increases,thusleading to small ripple current and turnoff current of the switch, and high efficiency is can be obtained. Meanwhile, the voltages of the capacitors are well regulated, thus achieving a tightly regulated output voltage. There exist two operating modes

MODE 1- Switch is ON

In the Fig.3 when switch S is turned ON, the input voltage source charges the inductor L. Meanwhile, C1 is in series with the voltage source and C2 to supply the load through S. When the switch S is ON the diodes D1 and D2 are OFF.

MODE 2- Switch is OFF

In the Fig.3 when S is turned OFF, the inductor charges C1

and C2 simultaneously, and the load is powered by Cf. In this converter, one of the SCs is the boost capacitor, and the other one is the buck–boost capacitor. The two capacitors C1 and C2 and the voltage source are connected in series to supply the load. The dotted lines indicate the devices which are turned off. When the switch S and Do is off, the voltage gain of the converter is high because part of energy is directly transferred to the load from the input source when the switch conducts. The efficiency of the converter is also high.



At steady state, the volt–second relationship of the inductor is given as:

TDVTDVVDTV cbbincbin )1()1)(( (2)

Where Vcband Vcbb are the voltages of the boost

capacitor and the buck–boost capacitor, respectively, and D is the duty cycle. Then, Vcb and Vcbband can be derived as:

)1( D

VV in

cb

(3)

)1( D

DVV in

cbb

(4)

The voltage gain of a high step-up converter with SIESC-SCs derived from Boost/Buck–Boost-based converter is

111

0

D

inV

D

inVV (5)

)1(

2

D

DinV

oV

(6)

Simulation of the DC-DC converter was done using MATLAB software.

C. Simulink model of DC-DC converter

If the systems input changes at any cause, the output of the

system must respond accordingly and change itself to reflect the

new input value. This is generally called as an Open loop system

(Non feedback system). It is a type of continuous control system

in which the output has no influence or effect on the control

action of the input signal.

Fig.4. Matlab-Simulink model of DC-DC converter.

The DC-DC converter consists of one switch(MOSFET), one

inductor(L), three capacitors namely C1,C2 and Cf. The input

voltage of the converter may vary from 25 to 36V, in open loop

system controller is not used. According to the input given the

output voltage may get varied. Assuming that, the DC output

from solar cell is 36V DC, the input voltage given to the circuit is

36V DC. The output obtained is 350V DC.The voltage and

current waveforms are also shown separately in Fig.5.



Fig.5. Simulation response of SCConverter.

Closed loop systems are mainly designed to obtain and maintain

the desired output condition automatically by comparing it with

the actual condition. This can be done by generating an error

signal which is the difference between the output and the

reference input. In other words, a ―closed loop system‖ is a fully

automaticcontrol system in which its control actions being

dependent on the output in some way.For any input voltage by

using PI controller the output is constant. For any Input for SC

converter is 36V, the output is 380V. The optimal kp, kivalue

designed by the PSO is 1.8307 and 0.0035433 respectively.

Fig.6. Matlab-Simulink model of DC-DC converter with PI controller.



Fig.7. Simulation response of SC converter using PI controller.

III.SOLAR CELL MODELLING

A Solar cell is an electronic device that converts the

light energy directly into electric energy without any form of

moving parts by using photovoltaic effect.

An equivalent electrical circuit is derived from the basic

concepts of solar cell. The equivalent circuits are shown in Fig.8

Fig.8. Equivalent Circuits of Solar Cell

Applying node equation in Fig.8, where IPH, diodes, RSE and RSH

are meeting together.

1exp

akT

qVIII opv

(7)

IIII SHDph (8)

SEDpv IIII (9)

)*(12

)(exp1

(exp 21

SH

SESEsat

SEsatPH

R

RIV

KT

IRVqI

kT

IRVqIII

(10)

Efficiency of solar cell depends many on the factors like shading on cells, Irradiance, Temperature etc. If the Solar Irradiance get decreased due to cloud, the earth movement or any other reason there will be reduce in the output current of the solar panel because of the IPH is directly proportional to the sun irradiance.But the temperature creates only small changes in the solar cell[9].

A. Simulation of a PV module

The PV module simulated in MATLAB in Simulink. For

simulation purpose all information can be found on the manufacturer PV module datasheet like open circuit voltage, short circuit current, the voltage at the MPP (Vmp), the current at MPP(Imp),the open circuit voltage/temperature coefficient (Kv),the short circuit current/temperature coefficient and the maximum experimental peak output power (Pmax).



Fig.9. P-V and I-V Characteristic of a PV array.

Maximum current is produced by a PV module when it‘s negative and positive and terminal is shorted, the maximum current obtained at this time is named as short circuit current of PV panel. When panel is short circuited, it‘s voltage across terminal is zero. When panel terminal is kept open circuited then the voltage across its terminal is maximum called open circuit voltage VOC of that panel.VOC is occurred when current is zero and ISC is occurred when voltage is zero on that curve and power of that panel at any point in Watt is calculated by multiplying both the current and voltage of that point.

IV.INTERFACING PV PANEL–CONVERTER

A. Control Schemes Controllers play vital role in the system for improved

reliability and précised output. Many controllers are used in closed loop system to achieve the desired action. PI controller is used here.

(i) PI Controller PI Controller will remove forced oscillations and steady state

error thus resulting in operation of ON-OFF controller and P controller respectively. The PI Controller takes into count the desired output of the converter and its actual output and thus finds the error. Based on this error, controller works to apply the input necessary to obtain the desired output. Better regulation is obtained. It has two terms Proportional and Integral terms. The current error value is proportional to the output value which is produced by the proportional term.The proportional response is adjusted by multiplying the error by a constant Kp, called the proportional gain constant.

The proportional term is given by:

errorpout VKP * (11)

The integral term is the sum of the instantaneous error over time and gives the accumulated offset that should have been

corrected previously. The accumulated error is added to the controller output after multiplying with the integral gain (Ki).

The integral gain is given by:

t

erroriout VKI0

(12)

The values for integral term (Kp) and proportional term (Ki) can be tunes manually or it can be programmed.PSO technique is used to tune the Kp and Ki values. (ii) Particle Swarm Optimization Approach

Particle Swarm Optimization (PSO) is a population based stochastic optimization structure developed by Dr.Eberhart and Dr. Kennedy, they get motivated by social behavior of bird flocking or fish schooling. Initially, they both started out developing computer software simulations of birds flocking around food sources, later they realized how well their algorithms worked on optimization problems. Over a number of iterations, a group of variables have their values adjusted closer to the member whose value is closest to the target at any given moment. Imagine a flock of birds circling over anarea where they can smell a hidden source of food. The one who is closest to the food chirps the loudest and the other birds swing around in its direction. If any of the other circling birds comes closer to the target than the first, it chirps louder and the others veer over toward him. This tightening pattern continues until one of the birds happens upon the food. It‘s an algorithm that‘s simple and easy to implement[10-12].The algorithm keeps track of three global variables: • Target value or condition. • Global best (gbest) value indicating which particle‘s data is

currently closest to the Target. • Stopping value indicating when the algorithm should stop if

the Target isn‘t found.

B. MPPT Technique MPPT algorithm is used for extracting maximum available

power from PV module under certain conditions. The voltage at which PV module can produce maximum power is called ‗Maximum Power Point‘ (or peak power voltage).Maximum power varies with solar radiation, ambient temperature and solar cell temperature. Maximum power varies with solar radiation, ambient temperature and solar cell temperature [13].

P&O algorithm is used to track the maximum power.According to P&O MPPT Technique the output voltage andcurrent from the module is considered and PV output powercalculated. Then the change in PV output power is calculated by subtracting the PV output power in one period with the PV output voltage in previous period. If both the change in voltage as well as the change in power are positive or both the change in voltage and current are negative the increase the module voltage. If one of them is positive and the other are negative then decreasethe module voltage.



Finally the output of the MPPT P&O algorithm is duty cycle for the converter.Lead Acid batteries were the first type of rechargeable batteries and are used in many applications. A Charge controller is necessary to ensure that the battery is not

over charged. The state of charge of the battery will also affect charge efficiency. For the input of 36 V in Matlab-Simulink alongwith PI controller based PSO technique, system output voltage is maintained constant at 380 V.

Fig.10. Matlab-Simulink model of PV array interfaced with proposed converter with battery charge controller

Fig.11. Simulation response of Battery charging

http://www.solar-facts.com/controllers



Fig.12. Simulation response of Proposed System.

Table 1. Parameters of Switched- Capacitor Converter.

Sl.No. Specifications Value

1. Input voltage 36V

2. Switching frequency 100kHz

3. Inductance 500µΗ

4. Capacitance 2000µF

5. Resistance 1460Ω

6. Output Voltage 380V

V.CONCLUSION A combination of the SC converter and Switching-

mode DC–DC converter has been proposed in this work for PV system applications. The operating mode of this proposed DC–DC converter adopting an SC cell has been analyzed. The turn-ON control of this converter charges the capacitors in series & supply the power to load and during turn-OFF period the energy stored in the inductor is released to charge the multiple capacitors in parallel and which increases the voltage gain of the DC–DC converter. The PI controller gain values have been obtained for differentoperating conditions using Particle Swarm Optimisation technique and their simulation results are plotted. The standalone PV system is also studied along with battery storage system for reliable operation.

REFERENCES

[1] K. Tseng and C. Huang, ―High step-up high-efficiency interleaved converter with voltage multiplier module for renewable energy system,‖ IEEE Trans. Ind. Electron., vol. 61, no. 3, pp. 1311–1319, March 2014.

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[3] S. Chen, T. Liang, L. Yang, and J. Chen, ―A boost converter with capacitor multiplier and coupled inductor for ac module applications,‖ IEEE Trans.Ind. Electron., vol. 60, no. 4, pp. 1503–1511, April 2013.

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[6] Y. Hsieh, J. Chen, T. Liang, and L. Yang, ―Novel high step-up dc–dc converter for distributed generation system,‖ IEEE Trans. Ind. Electron., vol. 60, no. 4, pp. 1473–1482, April 2013.

[7] Gang Wu, Student Member, Xinbo Ruan, and Zhihong Ye, Member, ―Non-isolated High Step-Up DC–DC Converters Adopting Switched-Capacitor Cell‖ IEEE Transactions on Industrial electronics, vol. 62, no. 1, January 2015

[8] Q. Zhao and F. C. Lee, ―High-efficiency, high step-up dc–dc converters,‖ IEEE Trans. Power Electron., vol. 18, no. 1, pp. 65–73, January 2003.

[9] R.Pon Vengatesh, S.Edward Rajan, ―Investigation of the Effects of Homogeneous and Heterogeneous Solar Irradiations on Multicrystal PV Module under various Configuration‖, IET Renewable Power Generation,Institution of Engineering and Technology, vol.9, No.3, Page No.:245-254, April 2015.

[10] Kennedy J., and Eberhart , R., ―Particle swarm optimization‖, Proc. IEEE International Conference on Neural Network, vol. 4, p. 1942 – 1948, 1995.

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[12] Shi Y., and Eberhart , R., ―A modified particle swarm optimizer‖, Proc. 1998 Int. Conf. Evolu-tionary Computation, The IEEE World Congress on Computational Intelligence. Anchorage, May 1998, p. 69 – 73

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International Conference on Computing Technologies (ICONCT ... · DC-DC converter is an electronic circuit that converts a DC source of a certain voltage level to another voltage